9 continent motors easily take up to 1500w with no problems. This is because once you get to speed, you will typically draw only about 1000w. So the 1500w is not really continous unless climbing steep hills.
I've not yet heard of a US vendor selling a 9 continent, or 9c clone kit that could not take 48v on the controller. They may not warantee it anymore, but inside the controller there is nearly always 63v capacitors, and the other components can take 48v fine as well.
Typically the "20 amp" controllers are actually 22 amp, and on a cycleanalyst you will see peaks of 1200w when you put 48v into one. Top speed should be right around 27 mph.
If you want a bit more speed, you can get into the 34 mph ballpark with a 72v 20 amp controller. Even at that wattage, around 1500w, you should have no problems melting the motor.
A 72v 40 amp controller would likely get you into the 40 mph club, or at least into the 39 mph club. But now with up to 3000w peak power, you can definitely melt down a motor fairly quick. To run above 2000w, I defintely recomend a temp sensor on the motor.